Reinventing the Wheel

At last year’s World Human-Powered Speed Challenge in Battle Mountain we found that our bike Eta is as efficient as we had predicted. We finished second overall and reached speeds of 126.3 km/hr, making Todd the 7th fastest human being in the world. We could have certainly gone faster had a series of mechanical issues, including broken spokes, blown tires, and clearance issues inside the bike-shell, prevented Eta from reaching full potential.

It’s one of our core beliefs that failure is a key part of success and so we are determined to use everything we’ve learned about Eta’s performance, capabilities, and shortcomings to hone her design for the 2015 challenge. One of our biggest set-back was broken spokes, so our key focus at the beginning of this year was to design new wheels that can handle the high loads, while maintaining low aerodynamic drag. Eta’s previous wheel design consisted of twenty thin carbon fibre-covered steel spokes which proved too few to handle the impact of bumps and the side loads from wind gusts. The spokes would snap at the threading, which was the weakest point in the structure.

Todd Reichert & Trefor Evans discuss wheel modifications.

The design of the wheel involves two primary criteria: the first is to provide a strong, stiff rolling structure that can withstand the loads. The second is to produce the least amount of aerodynamic drag, so that we can go as fast as possible. This year our wheel redesign started with the development of an accurate engineering model (or simulation) of the aerodynamics of various types of wheels. We combined relatively simple theoretical formulas with our “coast-down”data to be able to accurately predict the drag of almost any wheel, whether it be traditional spokes, a tri-spoke wheel or a disk wheel.

One of three custom made tri-spoke designs used to correlate the theoretical model to the coast-down tests.

Coast down test of a custom Tri-Spoke wheel.

The “coast-down” tests proved to be a lot of fun, and gave surprisingly accurate data. Since our wheel is completely enclosed inside the shell, it sees no oncoming air, and the only drag is the drag produced from spinning around at ultra-high velocities. This is very different from a traditional bike wheel, but it makes testing a lot easier. We clamp the wheel into it’s fork, spin it up to 160 km/hr with a grinder, and then use a magnet and sensor to measure it’s speed as it decelerates. From here we can back out exactly how much power is being lost at various speeds!

Coast down results for a subset of tires. The left graph shows the power absorbed by a single wheel spinning at a given velocity. The right graph shows the resulting drag coefficient. Solid lines represent test data, and dashed lines represent the results from the simulation.

With an accurate engineering model for drag prediction we can now do what we do best after years of building lightweight aircraft: aero-structural optimization. Yes, it is just as fun as it sounds! Aerodynamically, we want as few spokes as possible, but structurally, this just won’t be strong/stiff enough. There’s always a balance and the goal of aero-structural optimization is to find the perfect wheel that has exactly enough strength, while trading off between low weight and low drag to produce the fastest possible speed at Battle Mountain.

Outputs from the in-house aero-structural optimization program. The first two images show the exaggerated deflection of the wheel under a radial load and a side load caused by wind gusts. The second two images show the cross section of the spokes and the rim.

After 8 months of modelling, testing and design, we believe we have converged on the ultimate wheel for our tasks. It has five fairly thin, but very strong carbon fibre spokes, with a very thin rim to cut down on drag. So after all that work, how much faster are we going to be able to go? … Well, unfortunately it comes out to only about 0.5 km/hr faster than a more traditional disk wheel. Not exactly what we were hoping, but every km/hr counts.

Optimized penta-spoke wheel design.

As we have learned time and time again, every new innovation will certainly have teething problems, and we certainly don’t want to repeat the mistakes of last year. Our plan is to first build a set of disk wheels, which we know to be solid, reliable, and only marginally slower. These disc wheels, by definition, do not contain spokes. They are two solid carbon fibre sheets with light foam sandwiched in between, weighing about 1kg. Once we’ve built and thoroughly tested the disk wheels, we'll consider the option of building the optimal penta-spoke wheel.

Disk wheel with bonded rim, ready for hub installation and truing.

The other advantage of the disc wheel design is that they, essentially, contain many of the same components as the optimal pentaspoke wheel, so seeing how these components perform is time well spent. The most interesting of these components is the hub, which is very similar to those manufactured by HED, but is a custom design by Aerovelo. A unique feature of this design is a swapping mechanism that makes both wheels interchangeable as the hub can either be configured as the front wheel, which has the chain, or the rear wheel, which has the brakes (yes, there is only one brake... it’s not a stopping competition).

(Left) Hub installation with jacking screws used to true the wheel. The spline feature allows either the cog for the drive chain for the front wheel (Right), or an adapter to attach a disk brake for the rear wheel.

At the end of the day, we know we can build the "perfect" wheel. But starting with the disc wheels will off-set the risk of developing something new, but not as rigorously tested, that might fail us at the competition when it matters the most.

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